The modified Reynolds equation for non-wetting fluid flow through a rough-walled rock fracture

The no-slip boundary condition has usually been assumed to hold for the Reynolds equations (local cubic law) for fluid flow through rough-walled fractures. However, its validity for non-wetting fluid flow, such as prevails in fractured oil reservoirs, has been questioned. A series of experiments with a rough-walled fracture with mean aperture of 760 μm finds a higher flow rate for non-wetting fluid than wetting fluid. A modified Reynolds equation with a slip boundary condition is derived for non-wetting fluid flow through rough-walled fractures. Comparison of the modified Reynolds equation predictions with experimental results confirms that slip was a plausible explanation for a higher flow rate. The amount by which the flow rate for non-wetting fluids exceeds that for wetting fluids is found to depend highly on, and increase with, the degree to which the flowing fluid was non-wetted to thin immobile films on the surfaces. Numerical studies using the modified Reynolds equation indicate that the flow rate of non-wetting fluid became higher than that of wetting fluid as the roughness of the fracture increases. As the aperture becomes smaller, the flow rate ratio of non-wetting fluid to wetting fluid becomes large, leading to the endpoint relative permeability for the non-wetting fluid to exceed 1. The experimental and numerical studies clearly show that as the aperture of the fracture became less than a few hundred microns, the modified Reynolds equation with slip boundary conditions provides a better model for flow of a non-wetting fluid through rough-walled fractures.